1. Field of the Invention
The present invention relates to a zoom lens and an imaging apparatus, and more particularly to a zoom lens that enables an image shift by using a variable apex angle prism and an imaging apparatus using the zoom lens.
2. Description of the Related Art
In related art, as recording means for a camera, there has been known a method of converting an amount of light of a subject image formed on an imaging device surface into an electric output by means of a photosensor such as a charge coupled device (CCD), a complementary metal-oxide semiconductor (CMOS) or the like to record the converted electric output.
As microprocessing techniques have advanced in recent years, speeding up the central processing unit (CPU) and the enhancement of high-density-integration of the recording medium have been achieved, and it has been becoming possible to perform high speed processing of large size image data, which had not been able to handle until recently. Moreover, the enhancement of high-density-integration and the reduction in size have been attained for the light receiving element as well. As a result, recording of higher spatial frequency has been achieved, enabling the reduction in the whole size of a camera by the reduction in size of the light receiving element.
However, the light receiving area of individual photosensor has been reduced and the electric output thereof has been lowered owing to the enhancement of high-density-integration and the size reduction mentioned above, and an issue of increasing influences of noises due to lower electric output has been brought about. For addressing such an issue, the amount of light reaching the light receiving element has been increased by enlarging the aperture ratio of an optical system, or micro lens element (the so-called microlens array) has been disposed just before each element. The microlens array has restricted the exit pupil position of the lens system in compensation for introducing the light flux reaching a position between adjacent elements onto the elements. The reason of the restriction is that, if the exit pupil position of a lens system approaches the light receiving element, i.e. if the angle formed between the principal ray reaching the light receiving element and the optical axis becomes large, off-axis light fluxes advancing the peripheral portions of the image plane area forms large angles to the optical axis, and the off-axis light fluxes do not reach the light receiving element and causes the insufficiency of light.
As a zoom lens suitable for a video camera, a digital still camera and the like, each of which records a subject image by means of the photosensors, for example, a positive, negative, positive, positive four-group zoom lens has been known.
The positive, negative, positive, positive four-group zoom lens includes a first lens group having positive refractive power, a second lens group having negative refractive power, a third lens group having positive refractive power and a fourth lens group having positive refractive power, which are arranged in order from the object side. When the lens position arrangement of the four-group zoom lens changes from its wide angle end to its telephoto end, the first and the third lens groups are fixed in the optical axis direction, and a zoom operation is performed by the movement of the second lens group, and further the fourth lens group performs the operation of compensating the variation of the image plane position generated by the movement of the second lens group. A specific technique described in Japanese Patent Application Publication No. H06-337353 is known.
Now, because the angle of view at the telephoto end becomes narrow in a zoom lens having a large zoom ratio, such a lens has an issue such that even a minute camera shake causes a large movement of image.
As one of camera shake correction systems for correcting the movement of an image caused by the camera shake or the like, an optical camera shake correction system is known.
In the optical camera shake correction system, by combining a detection system for detecting a movement of a camera accompanying a camera shake caused by such as a shutter release, a control system for producing a displacement in a drive system based on a signal output from the detection system, the drive system for driving an optical system based on an output from the control system, and the optical system capable of shifting an image when the displacement is supplied from the drive system, the movement of an image caused by the movement of the camera can be cancelled and corrected by the movement of the image caused by the displacement of the optical system.
As the optical system to be used in the optical camera shake correction system, a lens shift system for shifting a part of a lens system in a direction perpendicular to an optical axis, a variable apex angle prism system for changing the apex angle of a prism disposed just before a lens system, and the like are known.
In such a lens shift system, there is an issue such that the number of lenses increases in order to suppress the change of optical performance that may occur at the time of shifting a predetermined lens, and it is difficult to reduce electric power consumption because the system requires to hold the lenses in the air (at a neutral position).
The variable apex angle prism system does not have the above-mentioned issue existing in the lens shift system.
The variable apex angle prism system may be classified into two categories, one in which a variable apex angle prism is disposed at a position nearest to the object side of the optical system, and the other in which a variable apex angle prism is disposed in the optical system.
As an example of disposing the variable apex angle prism at a position nearest to the object side, an optical system disclosed in Japanese Patent Application Publication No. S51-40942 exists.
In the case where the variable apex angle prism is disposed at a position nearest to the object side, there is a feature such that a change in the angle of view in relation to an amount of change in the apex angle does not depend on the focus distance of the zoom lens. When the movement of an image caused by a movement of a camera accompanying a camera shake or the like is corrected, the feature mentioned above has an advantage such that the apex angle can be controlled independently of the focus distance of the zoom lens. However, because the movement of the image is greatly caused in the telephoto state on the other hand, the technique disclosed in Japanese Patent Application Publication No. S51-40942 has an issue such that, in case of a large zoom ratio, stopping accuracy becomes extremely high in the telephoto state and improvement of the accuracy of the drive mechanism is required.
Moreover, because the variable apex angle prism is disposed on the object side of the first lens group having a large lens diameter, the diameter of the light flux entering the variable apex angle prism is large. As a result, there is an issue such that the variable apex angle prism itself becomes very large in size. Moreover, it is necessary to provide protection means such as disposing a protection glass in order that a hand of a user may not directly touch the variable apex angle prism. As a result, the miniaturization of the system has a limit.
On the other hand, as examples of having the variable apex angle prism in an optical system, those disclosed in Japanese Patent Application Publication Nos. S62-153816, H02-168223, H10-246855, H11-44845 and the like are known.
The optical system disclosed in Japanese Patent Application Publication No. S62-153816 places a variable apex angle prism at a part where light becomes a parallel light flux. The optical system disclosed in Japanese Patent Application Publication No. H02-168223 disposed a variable apex angle prism before the master lens group thereof. The optical systems disclosed in Japanese Patent Application Publication Nos. H10-246855 and H11-44845 severally dispose a variable apex angle prism in the neighborhood of an aperture diaphragm.
Because the on-axis light flux enters the variable apex angle prism as near to a parallel light in each of the optical systems disclosed in Japanese Patent Application Publication Nos. S62-153816, H02-168223, H10-246855, H11-44845, each of the optical systems has a feature such that the variation of an on-axis aberration, which occur when a prism angle is changed, can be suppressed.
A light flux which passes an optical system reaches an image plane while being refracted by lenses. A light flux toward the central portion of an image plane area reaches the image plane while being diverge or converge. Simultaneously, the principal ray toward the peripheral portion of the image plane area reaches the image plane while changing the angle formed with the optical axis.
Accordingly, when a variable apex angle prism is disposed in the optical system, it is possible to reduce a size of the system in comparison with the case where the variable apex angle prism is disposed on the object side to the first lens group. On the other hand, such a system has its issue such that a coma aberration may occur, a chromatic aberration may vary, or a trapezoid distortion may occur.
In particular, when the trapezoid distortion occurs, it is very difficult to correct image blur in the peripheral portion of the image plane area even if the image blur can be well corrected in the central portion of the image plane area, whereby the trapezoid distortion poses a critical issue. The trapezoid distortion tends to occur at a position where the principal ray forms a large angle with the optical axis when a light flux is not a parallel light.